Downsides of force_gl_names_reuse preventing adoption as default

[Rocco Tormenta]

Hi there, apologies if this has already been asked, I couldn't find
relevant information on the matter.

Recently I've come across some faulty applications (notably, old
Minecraft versions) that were calling `glBind*` functions with IDs of
-1, resulting in very slow performance due to the hash table defaulting
to full lookups.

Skimming through the source code I've noticed that a while ago, name
reusing/sparse names were implemented using idalloc (see MR
mesa/mesa!6600), locked behind a config flag. Indeed, setting the
force_gl_names_reuse=true environment variable fixes the performance issue.

The comment on the MR that added this feature mentions that this
behavior is in line with that of major proprietary drivers, so I was
wondering why this was added as an optional flag, instead of making it
the default behavior. I did find some potential reasons that could have
blocked the transition back when it was merged, though. For example,
more recently a commit was made (MR mesa/mesa!30106) lowering virtual
memory usage of the idalloc approach from 512MB to 512KB in the
worst-case scenario, so perhaps the higher memory footpring might have
discouraged adoption at the beginning. With such low requirements now,
however, perhaps it's worth reconsidering the default choice to benefit
from higher robustness against misuse of the API.

Is there anything I missed?

Rocco